There is a need for rechargeable battery systems which have a high energy density and hence are capable of storing and delivering large amounts of electrical energy per unit volume and/or weight, while also being capable of delivering high levels of peak power. Such high energy-high power battery systems have significant utility in a number of applications including military equipment, communication equipment, and robotics. However, energy density and power density of rechargeable batteries relate to each other in an inverse manner and this complicates the design of such rechargeable battery systems.
A high power density battery will always have a lower energy density than a battery optimized for high energy density. Power is related to the ability of a battery to discharge energy; therefore, release of energy in a high power battery is faster than in a high energy density battery. This high power load generates significant resistive heating which can create safety issues and impact the cycle life of the battery. Consequently, the cell size, cell geometry, and chemistries of high power batteries must be selected so as to allow for heat management. Consequently, high power batteries are not capable of storing and delivering as much energy per unit volume and/or size as are high energy density batteries.
On the other hand, high energy density batteries are optimized for delivering maximum energy per unit volume and/or weight and accordingly cannot safely deliver power at a rate comparable to that of high power batteries. Therefore, in order to achieve safe and reliable operation, a battery system must be maximized either for the delivery of high levels of power at a low energy density, or so as to have a high energy density and a low peak power capacity.
However, as noted above, practical applications often require a compact, high energy density power source which is also capable of providing high levels of peak power. Several approaches have been implemented in the prior art in an attempt to meet these conflicting goals. In one instance, the prior art has included a capacitor or ultra capacitor storage device in battery systems for accumulating and rapidly delivering high power pulses. This approach is limited insofar as the total amount of power which can be delivered by capacitor systems over long periods of time is relatively small, which greatly limits the utility of such systems. Furthermore, the sloping voltage profile of capacitor systems is not desirable when power has to be sustained over long periods of time. Typically, only 25% of the power stored in ultra capacitor based systems is practically usable. In another approach, a rechargeable high energy density battery system incorporates an auxiliary primary, non-rechargeable, battery. While such systems can meet particular power/energy profile needs, they are not practical for sustained operation since the primary battery must be replaced on a periodic basis. In other instances, the prior art has prepared hybrid, rechargeable battery systems which include a first group of cells optimized to deliver high power in conjunction with a second group of cells optimized to have a high energy density. Some such hybrid systems are shown in U.S. Pat. Nos. 7,399,554 and 7,635,541, as well as in U.S. Patent Application Publication 2007/0212596. Conventional wisdom had heretofore held that in hybrid battery systems of this type, voltages of the high energy and high power groups of cells must be identical so as to maintain proper operation of the system. Therefore, hybrid battery systems of the prior art were prepared utilizing generally identical battery chemistries for their high power and high energy segments. This necessity for matching of chemistries and voltages limits the design options for the hybrid battery system and furthermore requires the inclusion of charge/discharge control circuitry, all of which compromise the size, weight, and ultimate energy density of these hybrid systems. The constraint, in the prior art, of utilizing similar chemistries for power and energy electrodes has limited the options for hybrid battery systems, and is overcome by the present invention.
As will be explained hereinbelow, the present invention marks a break with the prior art insofar as it provides rechargeable hybrid battery systems which incorporate high power cells and high energy density cells having different voltages, charge/discharge characteristics, and chemistries. The differing cells of the present invention may also be designed with matching impedance and, as a result, the system of the present invention is inherently self-regulating with regard to charging and discharging and reduces or eliminates the use of charge controllers, switches, and the like. These and other advantages of the invention will be apparent from the drawings, discussion, and description which follow.